Examples of the power module substrate with a heat sink mentioned above include those proposed in Patent Documents 1 to 3.
In the power module substrates with heat sinks disclosed in Patent Documents 1 and 2, a power module substrate prepared by bonding metal sheets (a circuit layer and a metal layer) of Al (aluminum) to both surfaces of a ceramic substrate composed of MN (aluminum nitride), and a heat sink formed from aluminum are bonded together by brazing using an Al—Si-based brazing material.
Further, in a cooling device for a semiconductor module described in Patent Document 3, a structure is proposed in which a semiconductor module, which is prepared by bonding metal sheets (an upper electrode and a lower electrode) of Al (aluminum) to both surfaces of an insulating substrate composed of a ceramic material and also includes a semiconductor element bonded to the upper electrode, and a top plate of a cooling unit formed from aluminum are bonded together by brazing using a flux.
This brazing using a flux is a technique mainly used for bonding aluminum members together, and is a technique in which, for example, an Al—Si-based brazing material foil and a flux are positioned between the aluminum members, and the flux removes the oxide films formed on the surfaces of the aluminum members and promotes the melting of the brazing material, thereby achieving bonding.